National Guardsmen are put through riot training in Boston's Commonwealth Armory on Friday, October 18, 1974. Massachusetts Gov. Francis W. Sargent called up the guard to quell school violence, but the city has been relatively calm and the guard has remained in the armories. (Photo by JWG/AP Photo)

A man salts a downtown sidewalk in Washington, Thursday, March 5, 2015, as snow begins to fall. The U.S. federal government said its offices in the Washington area will be closed Thursday because of a new round of winter weather expected in the region. The Office of Personnel Management said non-emergency personnel in and around Washington were granted excused absences for the day. Emergency employees and telework-ready employees were expected to work. (AP Photo/Andrew Harnik)

A boy rides on a snow wagon towed by a robot dressed as a Teddy bear, which only moves forward by moving its legs, during the Ice and snow carnival at Taoranting park in Beijing February 9, 2015. (Photo by Kim Kyung-Hoon/Reuters)

Street vendors hold campaign posters for presidential candidate Patrice Talon ahead of the second round of Benin's presidential election on Sunday in Cotonou, Benin, March 18, 2016. (Photo by Charles Placide Tossou/Reuters)

A student shouts slogans against Venezuelan President Nicola Maduro during a protest on the main highway in Caracas on March 30, 2017. Venezuela' s Supreme Court took over legislative powers Wednesday from the opposition- majority National Assembly, whose speaker accused leftist President Nicolas Maduro of staging a “coup”. (Photo by Juan Barreto/AFP Photo)

Caoimhe Cooburn-Gray poses for a picture on St. Patrick's day in Dublin, Ireland March 17, 2016. Saint Patrick's Day is a cultural and religious celebration held on 17 March, the traditional death date of Saint Patrick (c. AD 385–461), the foremost patron saint of Ireland. Celebrations generally involve public parades and festivals, céilithe, and the wearing of shamrocks, as well as green or orange attire. (Photo by Clodagh Kilcoyne/Reuters)

A BASE jumper is pictured against the skyline shrouded in a thick haze during the Kuala Lumpur Tower International Jump in Kuala Lumpur, Malaysia, 02 October 2015. More than 100 BASE jumpers take part in this extreme sport event, which enters its 15th year. The haze hovering over Malaysia is caused by the ongoing plantation and forest fires in the nearby Indonesian provinces of Sumatra and Kalimantan. (Photo by Fazry Ismail/EPA)